For the 1999 NEC, tig [& mig?] welders are typically classed under Art.630 part B "Nonmotor-Generator." Duty Cycle likely 20-30% if fitted with 240V 50A cord cap. Electrically, {codewise} welders are their own breed when it comes to conductor/overcurrent-device sizing.
If the machine is listed/labeled, UL, etc. are fairly conscientious about reviewing installation instructions to make sure the welder manufacturer is not giving instructions that violate the NEC.
aldav, I'd go with the 6AWG.
[This message has been edited by Bjarney (edited 08-06-2002).]
